This invention relates generally to the laying of pipelines and in particular to a method for abandoning (or abandoning and recovering) a pipeline. The invention also relates to a system for use in such a method. Such a system and such a method are referred to herein as an A/R system and an A/R method.
During laying, it is sometimes necessary to abandon a pipeline and recover it later. For example, a pipeline might need to be abandoned because of severe weather conditions or mechanical problems aboard the laying vessel.
Conventionally, such abandonment and recovery is carried out by a method such as: welding an abandonment/recovery head onto the end of the pipeline being laid; connecting a steel rope to the head; transferring pipeline tension from the pipeline tensioning arrangement on the laying vessel that is used during normal laying to a winch on the vessel; and laying the pipeline and head on the sea bed. In severe weather conditions the vessel may also have to disconnect the rope from the winch and abandon the rope for later recovery; the end of the rope may then be marked by a buoy connected to it directly or via a pennant line.
Use of a conventional A/R system in deep water causes several problems.
The steel rope associated with the winch must have a very high tensile strength to support the high load resulting from the long length of pipeline, which initially extends from the seabed to the vessel. That requires a steel rope having a larger diameter, which, in turn, increases its weight. A higher load capacity is then required of the winch. The problem is increased because a very long steel rope and a very long pennant line are required, which creates further weight. The factors just described mean that the provision of a conventionally designed A/R system on a vessel able to operate in deep water is unduly expensive.
The effect of the factors referred to above is to make the provision of an effective and economical A/R system problematic when pipelines are being laid in deep water.
It is an object of the invention to provide a method and apparatus of laying a pipeline which overcomes or mitigates the above-mentioned problem.
According to a first aspect of the present invention there is provided a method of abandoning a pipeline being laid by a vessel, including the steps of reducing the tension at sea level of the sealine being laid by the vessel, and thereafter lowering the end of the sealine towards the seabed using a line from a winch on the vessel. Preferably, the tension is reduced by at least 15%. Advantageously, the tension is reduced by at least 30%, and more preferably by at least 40%.
By reducing the tension at sea level of the sealine being laid by the vessel before the winch is used to lower the sealine, it becomes possible to employ a winch whose maximum load bearing capacity is substantially less than the maximum tension under which the vessel is designed to lay the pipeline (the tension when the pipeline is being laid at the greatest depth). The maximum load bearing capacity of the winch may therefore be less than the maximum load bearing capacity of the pipe laying and tensioning arrangement that is used for laying the pipeline. Similarly the load bearing capacity of the winch line can be reduced thereby enabling its diameter to be reduced. In an example of the invention the load bearing capacity of the winch is reduced to about 50% of the maximum tension under which the vessel is designed to lay a pipeline. A reduction in the load bearing capacity (SWL) of the winch is critical in enabling an effective and economical A/R system to be provided even on a vessel designed to be capable of laying large diameter pipelines in deep water.
Where reference is made in the specification to a xe2x80x9csealinexe2x80x9d it should be understood that the term is referring to the pipeline laid on the seabed together with any pipeline or other elongate members extending upwardly from the pipeline.
In embodiments of the invention described below, the step of reducing the tension at sea level of the sealine being laid by the vessel comprises connecting one or more light elongate members to the end of the pipeline and lowering the elongate members into the sea. Where reference is made to a xe2x80x9clightxe2x80x9d elongate member, it should be understood that the member is to be light per unit length, when submerged in water, by comparison with the weight in water of the pipeline to which it is connected, so that the effect of connecting the light elongate member in the sealine is to reduce the weight of the sealine. It will be appreciated that any reduction in weight of the sealine extending between the vessel and the seabed will reduce the tension in the sealine at sea level. In the case where the pipeline is flooded, connection of a further length of pipeline sealed at both ends to prevent flooding represents connection of a light elongate member and reduces the tension in the sealine. Provided the water in which the method is carried out is not too deep, the use of such sealed pipeline or similar members may alone reduce the tension to a level that the winch is able to bear. In deeper water, however, even when the pipeline is not flooded, the tension in the pipeline may be too great for the winch to bear; in this case connecting a length of sealed pipeline will not reduce the tension and a lighter member is required to be connected; preferably such a member is sufficiently light that it is buoyant and, more preferably, generates an upward buoyancy force in water of the order of one half its weight in air or more. In especially deep water it may be desirable to connect together a series of buoyant members.
Said at least one buoyant elongate member may be composed principally of a lightweight material, for example, one selected from the group comprising glass fibre reinforced resins, other composite materials, carbon, titanium and aluminium. In an embodiment of the invention described below the material is a glass fibre reinforced vinyl ester resin. Parts of the elongate member, for example, end fittings, may be made of heavier material, for example, steel.
An intermediate elongate member is preferably connected between the end of the pipeline and a buoyant elongate member. The intermediate elongate member is preferably hollow and sealed against the ingress of water. The intermediate elongate member may be of sufficient weight not to be buoyant when empty. Preferably the member is provided with a valve to enable the interior of the intermediate elongate member to be flooded by sea water. Especially in the event that the sealine cannot be recovered by connecting a winch line to its end, the method may further include the step of cutting the intermediate elongate member, connecting the winch line to the end of the elongate member that is connected to the pipeline and recovering the end of the pipeline. The ability to flood the interior of the intermediate elongate member enables pressures to be equalised, thereby facilitating the cutting of the member.
When the one or more elongate members first begin to be lowered into the sea the tension that has to be applied to them is substantially the same as when laying pipe. It is a much preferred feature of the invention that the one or more elongate members are lowered using a pipe laying and tensioning arrangement on the vessel that is used for laying the pipeline; usually there is only one such tensioning arrangement although it may have several component parts. In order to facilitate use of the same tensioning arrangement, it is preferred that the diameter of the light elongate members is substantially the same as the diameter of the ordinary lengths of pipe making up the pipeline. Indeed the light elongate members are preferably of the same general dimensions as an ordinary length of pipe. As will now be understood, the invention enables the load bearing capacity of the winch to be substantially less than the load bearing capacity of the tensioning arrangement.
In some cases there will be no need to disconnect the winch line from the sealine because the laying vessel is able to remain at its station, connected via the winch line to the pipeline. In other cases, however, it will be desirable for the vessel to become completely freed and the method of the invention therefore preferably includes the subsequent step of disconnecting the sealine from the winch line. The disconnection can be effected in a variety of ways. A first possibility, if an ROV is provided as would normally be the case, is for the disconnection of the sealine from the winch line to be initiated by a physical contact of the ROV with a connector connecting the sealine to the winch line. It is also possible for disconnection to be initiated remotely in other ways, for example by a signal transmitted from an ROV or the vessel; the signal may be an acoustic signal. In that case the disconnection can be effected without the need for any physical interaction with an ROV or any other outside agency. In one advantageous embodiment of the invention the disconnection takes place automatically at a predetermined angle of the adjoining ends of the winch line and the sealine attached thereto. That predetermined angle is preferably in the range of 45xc2x0 to 75xc2x0 and in an embodiment of the invention is about 60xc2x0.
Even in a case where the disconnection is initiated by physical contact from the ROV, it is preferable that the physical contact amounts merely to a signal and that disconnection is able to take place with little or no intervention by the ROV or any other physical manipulation. Preferably the energy required for disconnection is stored in the connecting arrangement.
Preferably said at least one elongate member is connected to the pipeline through a connection which allows pivoting of the elongate member about a horizontal axis when the pipeline is lying horizontally. Such an arrangement also makes it easier to adopt the preferred method in which, after disconnection of the sealine from the winch line, an end portion of the sealine projects upwards from the seabed.
Thus, according to a second aspect of the invention, there is provided a method of abandoning a pipeline being laid by a vessel, including the steps of connecting one or more buoyant elongate members to the end of the pipeline, thereafter lowering the elongate members into the sea and then abandoning the pipeline, wherein one or more elongate members are so connected to the pipeline that the end of the elongate member at the end of the pipeline projects upwardly from the seabed, thereby facilitating the future recovery of the pipeline. Preferably, at least one elongate member is connected to the pipeline through a connection which allows pivoting of the elongate member about a horizontal axis when the pipeline is lying horizontally. Preferably, there is also a step in which the abandoned pipeline is recovered, for example, as described below with reference to the method according to the first aspect of the invention.
The method of the first aspect of the invention also preferably provides for recovery of the pipeline after abandonment. Thus the method preferably further includes recovering the abandoned pipeline by carrying out the further steps of raising the end of the sealine to the vessel using the winch, and raising the sealine further using a pipe laying and tensioning arrangement on the vessel that is used for laying the pipeline. As will be appreciated, in a case where the method includes disconnection of the winch line from the sealine, it is preferred that the further steps also include a preliminary step of connecting the winch line to the sealine.
The invention further provides an A/R system for carrying out a method of abandoning a pipeline as defined above with reference to either the first or second aspects of the invention. Such a system preferably includes a winch, a winch line and one or more light elongate members. Preferably the system includes one or more buoyant elongate members.
The A/R system preferably includes a connector for connecting the winch line releasably to the one or more buoyant elongate members. The connector preferably includes a stored energy arrangement for providing energy to actuate the release of the winch line from the one or more elongate members. The stored energy arrangement may take various forms including an electrical battery but preferably the energy is stored as pressurised hydraulic fluid in an accumulator and preferably the release of the winch line is hydraulically actuated. The hydraulic actuation may serve to extend or retract radially extending parts of a male element fitted inside a female element and unable to disengage when the parts are extended.
A joint may be provided for connecting the one or more buoyant elongate members to a pipeline. The joint preferably allows pivotal movement of the one or more buoyant elongate members about an axis transverse to the longitudinal axis of the end of the pipeline. Furthermore the joint preferably also allows pivoting of the one or more buoyant elongate members about the longitudinal axis of the end of the pipeline. By allowing such pivoting it becomes possible to arrange for the one or more buoyant elongate members to pivot upwardly from the part of the sealine resting on the seabed and therefore to enable a connection between the winch line and the buoyant members to be well clear of the seabed.
The invention still further provides a pipe laying vessel including an A/R system as defined in any of the paragraphs above. The vessel preferably includes a tensioning arrangement for withstanding the tension exerted by a pipeline during laying and a winch for use when abandoning the pipeline during laying, wherein the safe working load of the winch is lower than the safe working load of the tensioning arrangement.
The invention also provides a pipe laying vessel including an A/R system and comprising:
a tensioning arrangement for withstanding the tension exerted by a pipeline during laying,
a winch and a winch line,
wherein the safe working load of the winch is lower than the safe working load of the tensioning arrangement. Such a vessel may have any of the features referred to in the paragraphs above.
With a conventional winch system it would be undesirable to provide an abandonment/recovery system that was unable to cope in all the conditions that might be encountered because the SWL of the winch was not great enough. In the present invention, however, that reasoning is disregarded and a novel approach adopted.
The method and system employed in the preferred embodiments of the invention for disconnecting the winch line from the sealine is itself novel and may be applied in other situations apart from those referred to above. Thus according to a third aspect of the invention there is provided a method for disconnecting a winch line from a pipeline, the method including the steps of:
connecting a first part of a connector to the pipeline,
providing a second part of the connector on the winch line, the first and second parts being able to mate with one another and being provided with a locking/unlocking arrangement that can be remotely actuated,
mating the first and second parts of the connector and locking them together, and
when the first and second parts of the connector are underwater, remotely actuating the locking/unlocking arrangement to unlock the parts and thereafter disconnecting the parts.
Also according to the third aspect of the invention there is provided a disconnection system for disconnecting a winch line from a pipeline, the system comprising: a first part of a connector, the first part being connectable to a pipeline; and a second part of the connector, the second part being connectable to a winch line; the first and second parts being able to be connected and locked together by a locking mechanism and being able to be unlocked when underwater by remote actuation of the locking mechanism.
The method and system according to the third aspect of the invention may, except where inappropriate, include any of the features referred to above with respect to the first and second aspects of the invention. For example, the remote actuation may be automatic at a certain angle, may be initiated by a physical contact by an ROV or may be initiated by an acoustic signal. In an embodiment of the invention described below, the first and second parts of the connector include a male part and a female part, the male part being accommodated by the female part when the first and second parts are connected. In that embodiment, the first part forms the female part and the second part forms the male part (although it will be appreciated that the first part could be the male part and the second part could be the female part). Thus a significant proportion of one of the first and second parts may be inserted into and accommodated by the other. In particular, in the case where a stored energy arrangement is provided, it is preferred that that arrangement be housed in a section of one of the first and second parts that is, when the parts are connected, accommodated by the other of the parts, thereby enabling the disconnection system to be compact. The disconnection system preferably comprises a plurality of radially movable locking members being movable to and from positions in which the members lock the first and second parts together. The locking members may for example be moved, in use, by axial movement of a camming surface.